When Mitsu was a junior high school student, a fortuneteller gave her a strange piece of advice: You should marry the person you love the second most. Now an adult woman working at a clinic, she is married to a wonderfully kind and gentle husband named Ryota. He is exactly the kind of stable, loving partner the fortuneteller seemed to be recommending. Yet Mitsu has never been able to forget her first love, a man named Arishima. One fateful night, she runs into him unexpectedly, and the two begin an intimate relationship, starting an affair conducted in secret from their respective spouses.

The story follows these four individuals as their two married lives become entangled in a web of mixed emotions. Arishima has a wife named Reika, who is a picture-perfect mother and devoted spouse, as well as a newborn child. What initially begins as what Mitsu and Arishima tell themselves is a no-strings-attached affair gradually begins to change both of them. As they continue to meet, their feelings deepen, and the simple arrangement becomes increasingly complicated.

Back in their respective homes, the tension builds. Mitsus husband Ryota begins to suspect that something is wrong, and in response, he intensifies his devotion to her in an attempt to win her back and save their marriage. Arishimas wife Reika faces the emotional turmoil of her husbands betrayal. The narrative shifts between the perspectives of all four characters, showing how the affair affects each person differently. The series covers the progression of this four-way relationship from the initial reconnection, through the period of secret meetings, and into the unavoidable consequences when secrets begin to surface. The story ultimately explores whether a marriage can survive such a betrayal and whether Mitsu will ever find out what it truly means to love someone just a little bit.